The drive for modern motor vehicles is increasingly involving the use of electric machines as a sole drive or in unison with a drive of another type (hybrid drive). The electric machine is normally actuated by means of power electronics, which comprise an inverter that takes the DC voltage from a high-voltage battery on board the motor vehicle and produces an alternating current. Such inverters are frequently controlled on a field-oriented basis. Such control is also called vector control. In this case, provision may be made for a space vector (for example a current vector) to be moved that rotates with the driveshaft of the electric machine. In other words, this converts the phase currents used for actuating the electric machine into a coordinate system that is fixed with respect to the rotor and that co-rotates with the magnetic field of the machine. Such a coordinate system is frequently referred to as a dq system. In the case of field-oriented control, the current components Id and Iq transformed in this manner are then controlled instead of the phase currents. Iq can also be referred to as a torque-forming setpoint current value and Id can also be referred to as a field-forming setpoint current value in this case. In the case of a separately excited synchronous machine, an additional field-forming setpoint current value (Ie) can also be used.
A field-oriented actuation method is disclosed in DE 10 2010 061 897 A1, for example.
In the case of field-oriented control systems, a setpoint current value forming unit is normally used. This receives a setpoint torque value as an input signal. Said setpoint torque value is ultimately prescribed by the driver by means of the gas pedal, the position of which is sensed and converted into a setpoint torque value. This involves any maximum values and maximum gradients being taken into account in order to prevent overloads on the electric motor or oscillations.
The setpoint torque value needs to be provided by the motor controller. For this purpose, said motor controller computes a torque-forming setpoint current value and at least one field-forming setpoint current value so as to control the electric motor on a field-oriented basis. The torque-forming setpoint current value and the at least one field-forming setpoint current value are usually coordinated with one another such that an optimized operating point is obtained by which the electric power is converted into mechanical energy as efficiently as possible. For this purpose, the setpoint current value forming unit can either have online optimization with respect to the respective operating point, i.e. the operating point is recalculated each time, or it can use stored tables or formulae to query optimized operating points offline. The computation complexity for both methods is not negligible in this case, as a result of which the computation is executed relatively slowly in relation to the speed of the field-oriented control. In normal driving states, such as acceleration and braking, the computation speed is totally adequate, however, especially since sudden changes in the setpoint torque value characteristic need to be avoided in order to avoid bucking in the drive train, which is suspended so as to be capable of oscillation.